The invention relates to a method for operating an image system of an imaging medical examination device, the image system having a reception unit for receiving a plurality of signals arising at different locations, and a display unit for the imaging representation of pixels, the pixels each being assigned at least one signal. The invention additionally relates to an imaging medical examination device having an image system, the image system having a reception unit for receiving a plurality of signals arising at different locations, and a display unit for the imaging representation of pixels, the pixels each being assigned at least one signal.
In medical X-ray technology use is made of digital imaging systems having a reception unit with a digital image converterxe2x80x94instead of an analog image converter. Such a digital image converter acquires an image comprising a plurality of pixels. An individual pixel can be generated e.g. by the signal of an individual element of a photodiode array, of a CCD image converter or an amorphous silicon detector (a:Si detector). The individual elements receive light signals arising at different locations and image them on a display unit in an imaging fashion.
Image converters of this type can be affected by pixel failures caused e.g. by the failure of an individual converter element. There may also be pixel failures in groups (so-called clusters), which lead to the failure of entire lines or columns and are caused for example by interruptions in the address lines.
The failure of one or more pixels or measurement channels can lead to image artifacts of greater or lesser severity, which become visible for example as black rings in a computer tomograph image. If every image converter affected by such an artifact were discarded from the series during the production of a digital image system, this would lead to a high reject rate. On the other hand, however, given the multiplicity of pixels present, it is not necessary for the signal of every individual measurement channel to reach the imaging stage. In order to decrease the rejection of detectors, it is known, therefore, for example from DE 195 27 179 C1 or from DE 195 27 148 C1, to correct a defective pixel. For this purpose, the procedure is performed in two steps: firstly a defect determination takes place, which yields information about which pixels are defective and which are good. When this information is present, the defective pixels can be corrected in a second step. The correction can be achieved for example by replacing the defective pixels by linear interpolation of adjacent pixels. For the correction of column or line defects, DE 195 27 179 C1 discloses proceeding separately according to line defects in a first correction step and according to column defects in a second correction step.
A correction circuitxe2x80x94based on an interpolation methodxe2x80x94for correcting defective pixels in an image system or a CCD apparatus is also disclosed in EP 0 687 106 A1.
Proceeding from an image system with a potentially defective pixel, the invention is based on the object of specifying a method for operating such an image system in an imaging medical examination device which improves the reliability of the operation of the image system. An imaging medical examination device having an image system is also intended to be specified for the same purpose.
The first-mentioned object is achieved, relative to a method of the type mentioned in the introduction, by virtue of the fact that an event of the undisturbed operation of the medical examination device automatically triggers a defect determination for determining a defective pixel possibly present in the image.
The defect determination can be performed as it were on line during use and not just once after the production of the digital image sensor or during the calibration thereof. This makes it possible firstly for imminent defects already to be identified at a point in time at which the image is not yet severely disturbed. Secondly, it is thus possible to effect continuous correction of newly occurring defects, so that even over a long period of operation, the quality of the images recorded by the image sensor having an increasing number of defects is not impaired or is virtually unimpaired.
In particular, an event that occurs anyway during operation is used or a triggering event is generated during the undisturbed operation.
In this case, the invention is based on the insight that in image systems of future medical examination devices, defective pixels will have to be reckoned with to an increasing extent, since the number of detector channels will rise. It will increasingly happen that such image or channel failures will not occur until during clinical operation and interrupt the latter or at least cause massive disruption thereto. Although occasionally the defects may arise as early as during the production of the digital image converter, there is nonetheless the risk that further defects or indeed the first manifestation of defects will occur during the use and operation of the digital image converter, that is to say e.g. during clinical use, and interrupt the latter or at least cause massive disruption thereto. In the case of the method according to the invention, such defects and, consequently, an operational disruption are counteracted since the defect determination is triggered automatically. This means that it is possible, in particular, for an imminent defect already to be detected at a point in time at which the image is not yet severely disturbed, in order to initiate suitable countermeasures before the occurrence of a later severe disturbance. The method additionally has the advantage that separate intervention by an operator is not necessary to trigger the defect determination. Rather, the defect determination can take place without the action and even without the knowledge of an operator.
According to a preferred embodiment, the triggering event is derived from an operating process which does not serve for the defect determination, in particular from an operator""s control process which does not serve for the defect determination.
Preferably, the triggering event is derived from a switch-on process performed on the medical examination device. By way of example, a defect determination sequence or a defect determination algorithm is automatically triggered when the medical examination device is switched on.
It is likewise preferred for the triggering event to be derived from a calibration process performed on the medical examination device. Such a calibration process is carried out e.g. when the examination device is switched on or during the operation of said examination device by an operator, e.g. by a doctor. During this calibration process, the image channels or pixels are calibrated individually and immediately examined with regard to a defect during this opportunity.
Preferably, the triggering event is generated at a defined point in time before, during or after an image acquisition procedure, in particular before, during or after a patient examination or a scan.
The triggering event can also be generated by a counting process. In particular, the counting process counts a process which is repeated during operation of the medical examination device, in particular a switch-on process, a calibration process and/or examination process. A trigger signal as the triggering event is triggered for example whenever the counting process has continued counting by a constant interval. This affords the advantage that a defect determination is automatically triggered whenever the image system has been exposed to a high load and, accordingly, it is with increased probability that the occurrence of defects is to be reckoned with.
It is likewise preferred for the triggering event to be generated by a time measuring process. For this purpose, a corresponding trigger signal can be derived for example from a clock generator or a timer of a computer which controls the examination device. By way of example, a trigger signal or trigger event is generated at fixed time intervals, for example hourly.
According to an especially preferred refinement, after the defect determination, a correction process is automatically triggered if a defective pixel was detected. As a result, outage times of the examination device in clinical operation are avoided to the greatest possible extent. Image artifacts become visible only to a very small extent or only sporadically.
Preferably, during the correction process, the assignment of the defective pixel to its signal is canceled and, instead of this, the pixel is assigned one or more signals of one or more other pixels. By way of example, interpolation from the signals of adjacent pixels takes place.
The above-described correction process by newly assigning one or more signals of one or more other pixels to a defective pixel leads to complete elimination of an image artifact. Such a correction process is carried out as sole correction measure in particular in the edge area of a computer tomograph image, since, in the edge area, the failure of the information from a single pixel does not lead to significant impairment of the meaningfulness of the image.
The automatic performance of a correction process has the advantage that the service costs for the examination device are significantly reduced since the attendance of service personnel will in many cases not be necessary.
The correction process is, in particular, an interpolation process in which interpolation is effected, for example linearly, between pixels adjacent to a defective pixel. By way of example, the correction process according to the patent claims of DE 195 27 179 C1 is employed. It is also possible to use the correction measures described in EP 0 687 106 A1, in particular as set forth therein in the claims.
The defect determination takes place, in particular, according to a method as disclosed in the patent claims of DE 195 27 148 C1.
The medical examination device is, for example, a computer tomograph, a magnetic resonance imaging scanner or a conventional X-ray apparatus, e.g. a radiography apparatus.
In one specific embodiment of the method, it is provided that in connection with the defect determination after carrying out a first correction process in which already known image defects are corrected, the corrected image is analyzed in order to determine further defects or defects that are still present, which are corrected in a second correction process. Thus, a two-stage correction takes place.
In the specific embodiment mentioned, the method according to the invention advantageously provides a two-stage correction of an image currently being recorded, or of a chronologically previously recorded image loaded from a memory. First of all, in a first correction process, a first correction of the image or of the image signals is effected in order to correct already known defects which were determined e.g. as early as after the production of the reception unit in the context of serviceability tests. After this correction process, an image is present which has been corrected for the first time and is already largely free of defects on account of the correction. In order then to identify new defects, the already corrected image is subsequently analyzed. Depending on whether or not further defects are then identified, the image is either corrected in a second correction process in order to eliminate the new defects, if such have been detected. If no further defects are present, then the image can be processed further and output.
What is effected here, then, is firstly a correction of the defects known as it were xe2x80x9coff linexe2x80x9d and, in addition, a correction of the defects ascertained xe2x80x9con linexe2x80x9d, on account of the defect analysis, performed according to the invention, of the image currently being recorded. The correction is thus effected with regard to the defects that are actually present in the image, and not just with regard to the already known defects that were indeed determined earlier and, if appropriate, may only constitute a portion of the overall defects.
The image itself can be filtered after the first correction process, after which the filtered image is first analyzed, in which case e.g. a median filter or a high-pass filter can be used as the filter. The defect-free image areas are filtered out in the context of this filtering, with the result that only image areas that are possibly still affected by defects are visible in the filtered image on which the analysis is based.
According to the invention, in the context of the analysis, the pixel-related signals may then be compared with one or more threshold values. A defect may be identified in the context of this analysis e.g. from the fact that the signal falls below a threshold value or, alternatively, that the signal or the signal noise exceeds or falls below a specific threshold value. A defect can also be identified using already filtered signals in the manner described. In a development of the invention, this analysis result may then be used to generate a new defect map, which describes the detected defect or defects that is or are new or still present, and which is used to effect the correction in the second correction process. In this casexe2x80x94like the first correction previouslyxe2x80x94the correction can be effected by means of any known correction method, e.g. by means of the correction methods already described in the documents DE 195 27 179 C1 or DE 195 27 148 C1 mentioned in the introduction.
In this case, in the context of the first correction process, the image may be corrected using an old defect map, which describes already known defects. This old defect map is used as a basis for the first correction process; it specifies the position of the already known defects, as were determined e.g. after the production of the reception unit or in the course of earlier calibration, thereby enabling a targeted defect correction in the context of the first correction process.
Since the newly detected defects are normally permanent defects which would likewise occur again in subsequently recorded images, it has proved to be particularly expedient if the old defect map is updated using the new defect map. In other words, the newly determined defects are concurrently included in the old defect map, and the latter is thus adapted to the given defect situation and updated, with the result that, in the context of the first correction process of a subsequently recorded image, both the new defects already known at the time and the new defects determined in the last detection and correction step are immediately corrected. The old defect map is thus continuously adapted to the actual defect state.
In this case, the updating can be effected only when one or more defects that are newly present or still present are detected. This advantageously avoids the situation in which a step of updating the old defect map takes place after each new defect determination even when no new defects have been determined.
Finally, it may be provided that a flat-fielding correction of the image is effected in the context of the first correction process. In the context of this known flat-fielding correction, firstly the recorded image is corrected using an offset image, by means of which the originally provided offset of the digital reception unit is corrected. Furthermore, a correction is effected using a gain image, which takes account of the different gain factors of the individual pixels. This flat-fielding correction is intrinsically known per se and need not be discussed in any further detail.
Moreover, the method is configured in a particularly preferred manner in that after the defect determination, a message is automatically sent via a data link to a service device if a defective pixel was detected. This affords the advantage that the service device is continuously informed about defects that occur on the examination device. It can then decide remotely, for example, whether a correction process that may already have been triggered automatically suffices to eliminate the defective pixel, or whether further-reaching measures should be initiated, e.g. replacing the reception unit.
When the defect determination is carried out, a pixel is detected as defective preferably by virtue of the fact that the assigned signal falls below a minimum value and/or the noise in the assigned signal exceeds a maximum value. The determination of the noise, in particular, enables imminent defects to be identified in good time.
The defect determination can also be carried out on a stored image. This is advantageous in particular if the defect determination is triggered by an event at the instant of whose occurrence no image data are regularly present, such as e.g. if a trigger signal is generated at fixed time intervals.
The apparatus-related object is achieved, relative to the medical examination device mentioned in the introduction, by means of a detection device for automatically determining a defective pixel possibly present in the image, in which case the detection device can be activated by an event of the undisturbed operation of the medical examination device.
Such a medical examination device is particularly suitable for carrying out the method according to the invention. When the detection device is activated, the defect determination can be triggered in said detection device.
The advantages and configurations mentioned with regard to the method apply analogously to the medical examination device according to the invention.
Preferably, the detection device can detect a pixel as defective if the assigned signal falls below a minimum value and/or if the noise in the assigned signal exceeds a maximum value.
According to a particularly preferred refinement, the examination device has a correction device for automatically eliminating a defective pixel that has possibly been detected, in which case the correction device is connected to the detection device and can be activated by the latter if a defective pixel is detected.
Such a preferred examination device is distinguished for example by the fact that the detection device is designed for analysis of the image that has been corrected by means of the correction device a first time with regard to already known defects, for the purpose of determining defects that are new or still present, and the correction device is designed for renewed correction of the corrected image with regard to the defect or defects that is or are new or still present.
In this case, the detection device has a filter for filtering the image after the first correction process and an analysis means for determining one or more defects that is or are new or still present, the filter expediently being a median filter or a high-pass filter.
The analysis means itself may be designed for comparing the pixel-related signals with one or more threshold values for the purpose of determining a defect. Furthermore, the analysis means or the detection device may be designed for generating a new defect map, which describes the detected defect or defects that is or are new or still present, and the correction device may be designed for correcting the image in the second correction process using the new defect map. Finally, the correction device may be designed for correcting the image using an old defect map, which describes already known defects, in the context of the first correction process.
Furthermore, it is expedient if the detection device or the correction device is designed for updating the old defect map using the new defect map. In this case, the respective defect maps may be stored either in the detection device or the correction device, that device where the defect maps are stored expediently carrying out the corresponding updating.
Finally, the correction device may be designed for carrying out a flat-fielding correction of the image in the context of the first correction process.
It is additionally preferred for the detection device to have a data interface for sending a message to a service device, in which case the message can be sent automatically by the detection device if a defective pixel is detected.
The detection device may also be connected to an image memory, from which it is possible to retrieve an image which was generated by the image system at an earlier point in time.